A rolling circle model of the in vivo replication of bacteriophage phiX174 replicative form DNA: different fate of two types of progeny replicative form

In a preceding paper (Schröder and Kaerner, 1972) a rolling circle mechanism has been described for the replication of bacteriophage φX174 replicative form. Replication involved nicking and elongation of the viral (positive) strand component of the RF molecule resulting in the displacement of a single-strand tail of increasing length. The synthesis of the new complementary (negative) strand on the single-strand tails appears to be initiated with considerable delay and converts the tail into double-stranded DNA. Before the new negative strand is completed the replicative intermediates split into (I) a complete RF molecule containing the “old” negative and the new positive strand, and (II) a linear, partially double-stranded “tail” consisting of the complete old positive strand and a fragment of the new negative strand.The present study is concerned with the fate during RF replication of these fragments of the rolling circles. Those RFII molecules containing the old negative strands appear to go into further replication rounds repeatedly. Some of the tails were found in the infected cells in their original linear form. “Gapped” RFII molecules, which have been described earlier by Schekman and co-workers (Schekman &; Ray, 1971; Schekman et al., 1971), are supposed to originate from the tails of rolling circle intermediates by circularization of their positive strand components. Evidence is provided by our experiments that even late during RF replication these gaps are present only in the negative strands of RFII. Appropriate chase experiments indicated that the tails finally are converted to RFI molecules. Progeny RFI molecules could not be observed to start new replication rounds under our conditions although we cannot exclude that this might happen to some minor extent.The results presented suggest that the master templates for RF replication are the first negative strands to be formed, rather than the parental positive strands.